Added explicit exception raise when no stack found. (#3559)

* Added explicit exception raise when no stack found.

Currently, any operation that uses 'get_stack' method from 'CloudFormationBackend' class
will fail with AttributeError or jinja2 exception if ran against non-existing stack(created/deleted)
To fix the issue I explicitly raised a 'ValidationError' exception.
Added tests for boto and boto3 responses.

* Moved non-existing stack tests to 'test_stack_events'

When using 'update_stack' to test raising an exception when the stack doesn't exist
test coverage dropped by 0.5%. I am using stack_events instead.

* Removed some unreachable paths

After adding the exception couple of paths in the code are unreachable as 'get_stack' doesn't return 'None' anymore.
This is the reason why coverall was reporting decreased coverage.

* Removed an unreachable path I missed

* Added couple of tests in cloudformation/models

* Added more assertions around raised exception

* Formatted document using black to fix issue with travis.

tests/test_cloudformation/test_cloudformation_stack_crud.py

@@ -570,6 +570,15 @@ def test_describe_stack_events_shows_create_update_and_delete():
 
     list(stack_events_to_look_for).should.be.empty
 
+    with pytest.raises(BotoServerError) as exp:
+        conn.describe_stack_events("non_existing_stack")
+    err = exp.value
+    err.message.should.equal("Stack with id non_existing_stack does not exist")
+    err.body.should.match(r"Stack with id non_existing_stack does not exist")
+    err.error_code.should.equal("ValidationError")
+    err.reason.should.equal("Bad Request")
+    err.status.should.equal(400)
+
 
 @mock_cloudformation_deprecated
 def test_create_stack_lambda_and_dynamodb():

tests/test_cloudformation/test_cloudformation_stack_crud_boto3.py

@@ -255,6 +255,7 @@ def test_boto3_filter_stacks():
     conn.create_stack(StackName="test_stack", TemplateBody=dummy_template_json)
     conn.create_stack(StackName="test_stack2", TemplateBody=dummy_template_json)
     conn.update_stack(StackName="test_stack", TemplateBody=dummy_template_json2)
+
     stacks = conn.list_stacks(StackStatusFilter=["CREATE_COMPLETE"])
     stacks.get("StackSummaries").should.have.length_of(1)
     stacks = conn.list_stacks(StackStatusFilter=["UPDATE_COMPLETE"])
@@ -318,6 +319,18 @@ def test_boto3_describe_stack_set_operation():
 
     response["StackSetOperation"]["Status"].should.equal("STOPPED")
     response["StackSetOperation"]["Action"].should.equal("CREATE")
+    with pytest.raises(ClientError) as exp:
+        cf_conn.describe_stack_set_operation(
+            StackSetName="test_stack_set", OperationId="non_existing_operation"
+        )
+    exp_err = exp.value.response.get("Error")
+    exp_metadata = exp.value.response.get("ResponseMetadata")
+
+    exp_err.get("Code").should.match(r"ValidationError")
+    exp_err.get("Message").should.match(
+        r"Stack with id non_existing_operation does not exist"
+    )
+    exp_metadata.get("HTTPStatusCode").should.equal(400)
 
 
 @mock_cloudformation
@@ -1136,6 +1149,16 @@ def test_delete_change_set():
     cf_conn.delete_change_set(ChangeSetName="NewChangeSet", StackName="NewStack")
     cf_conn.list_change_sets(StackName="NewStack")["Summaries"].should.have.length_of(0)
 
+    # Testing deletion by arn
+    result = cf_conn.create_change_set(
+        StackName="NewStack",
+        TemplateBody=dummy_template_json,
+        ChangeSetName="NewChangeSet1",
+        ChangeSetType="CREATE",
+    )
+    cf_conn.delete_change_set(ChangeSetName=result.get("Id"), StackName="NewStack")
+    cf_conn.list_change_sets(StackName="NewStack")["Summaries"].should.have.length_of(0)
+
 
 @mock_cloudformation
 @mock_ec2
@@ -1309,6 +1332,19 @@ def test_stack_events():
 
     list(stack_events_to_look_for).should.be.empty
 
+    with pytest.raises(ClientError) as exp:
+        stack = cf.Stack("non_existing_stack")
+        events = list(stack.events.all())
+
+    exp_err = exp.value.response.get("Error")
+    exp_metadata = exp.value.response.get("ResponseMetadata")
+
+    exp_err.get("Code").should.match(r"ValidationError")
+    exp_err.get("Message").should.match(
+        r"Stack with id non_existing_stack does not exist"
+    )
+    exp_metadata.get("HTTPStatusCode").should.equal(400)
+
 
 @mock_cloudformation
 def test_list_exports():